In general, eye tracking systems are utilized in various applications to detect an individual's visual fixation (or gaze point) on a display screen and/or track the individual's eye motion relative to the individual's head. An individual's eye movement includes fixations and saccades. The term “fixation” refers to the maintaining of the visual gaze on a single location, i.e., when eye gaze pauses in a certain position. By way of example, a fixation can have a duration in a range of about 100 ms (when reading text) to about 350 ms when viewing a scene. The term “saccades” refers to quick, simultaneous movements of both eyes in the same direction.
An eye tracking system is typically implemented using an optical-based eye tracking device to detect fixations and measure eye movement. In a typical optical-based eye tracking system, infrared light is emitted towards a user's face, and the infrared light reflected from the user's eyes is captured by a video camera or some other application specific optical sensor. Depending on the application, some video-based eye tracking devices are configured to capture features such as corneal reflection and the center of the pupil, while other eye tracking systems are configured to capture double reflections from the front and back of the retina, for example. The captured information is then analyzed to determine eye movement and gaze direction based on changes in reflections.
Optical-based eye tracking systems are used to estimate gaze direction in various research studies such as cognitive linguistics and product design, for example, in which tracking a user's gaze location on a computer screen provides useful information. For example, gaze tracking is implemented in usability studies for computer user interfaces, wherein web designers and other application designers are interested to know what displayed features of a web page or user interface are viewed (or not viewed) by users, and to understand how users perform complex computer tasks or otherwise interact with an application user interface.
It is understood that eye tracking systems do not actually determine an absolute gaze direction or fixation location on a display screen, and can only measure changes in gaze direction. To accurately estimate a user's gaze direction or fixation location on a display screen, eye tracking systems implement a calibration process to provide an accurate mapping from eye position to display location. In general, a calibration procedure involves displaying a series of calibration items at different locations across a computer display, and recording data values that correspond to each gaze position of the user as the user looks at each displayed calibration item. While a set of calibration data can be effective to process eye tracking data and determine gaze points of a user on a display screen, systematic calibration errors are introduced into recorded gaze point locations for eye tracking data when the user changes his or her head position relative to the display after calibration is completed, which can render the captured data unusable.